you get the value of the combo box with its own inbuilt method getValue, eg temp = myComboBox.getValue(), then you would use LoadVars to send it to a php script, like
ActionScript Code:
myVar = myComboBox.getValue();
myLV = new LoadVars();
myLV.myVar = myVar;
myLV.send("myPHP.php");
(you could put this code into a function and make it the onchange handler of the combobox or the onclick handler of a "submit" button) which would be just the same as sending a form to that page and the variable myVar would be available in the php _POST object at the other end:
PHP Code:
<?php
echo $_POST["myVar"];
?>